In normal, adult rats, naive PC-12 cells,implanted into brain disappear while those differentiated by ras-oncogene, persist for at least 8 weeks in the same brain. To determine whether the naive cells are immunologically rejected or, as suggested by others, migrate from the implantation site. Naive cells have been implanted into brains of adult athymic, immunologically deficient, nru nude rats. The naive PC-l2 cells not only persist by for large tumors within 2-3 weeks. Naive PC-12 cells, therefore, disappear from mature normal rat brain because they are immunologically rejected. The ras-PC-12 cells persist because their major histocompatibility complexes (MHC) might have been altered by the oncogene. Antibodies specific for MHC, both classes I and II, immunostain naive PC-12 cells and those differentiated by nerve growth factor or by ras-oncogene. Flow cytometry is to be used to estimate any differences in MHC expression that could account for host tolerance to the ras cells. To further define ras differentiation, in terms of adhesion properties that may influence synapse formation, endogenous lectins of naive and ras-PC-12 cells have been identified, tentatively, by an erythrocyte agglutination test as a lectin specific for glucose -containing oligosaccharides.